The effect of soil boundary conditions in fully coupled mathematical models of partly saturated vegetated soil poromechanics

This study investigates the interactions between growing root systems, precipitation, and the physical properties of the underlying or foundation soil on the spatiotemporal porewater pressure, water saturation, and shrink-swell response of partly saturated soils. Physical processes (including water flow and retention, and hydro-mechanical interactions) within the vegetated soil layer are described through fully coupled poroelastic equations previously validated by field measurements. Nondimensionalisation is used to explore the significance of these processes at different temporal and spatial scales. Six representative two-layer soil scenarios are then examined to investigate the effect of groundwater level and hydraulic conductivity of the foundation soil on the basal boundary conditions in simplified one-layer models. These scenarios are classified as (i) deep foundation soil, (ii) a deep single soil layer, (iii) low permeability foundation soil, (iv) shallow foundation soil, (v) high permeability foundation, and (vi) soil layers of comparable characteristics. Following conditioning without rainfall or plants present, simulations are carried out for conditions of (i) no rainfall and no plants, (ii) rainfall only, (iii) plants only, and (iv) rainfall and plants together. The simulations show significant sensitivity between the root function and the basal boundary condition, with the root water uptake and bottom flux rates critically dependent on porewater pressure fluctuations. Results also show that root water uptake maintains the magnitude of porewater pressure and deformation cycles around the conditioning simulation values, highlighting the key role of plants in maintaining the safety and serviceability of geotechnical infrastructure assets.